How long does it take for 87.5 % of each isotope to decay? (The isotopes are below)
Krypton-73= 27 seconds
Krypton-74= 11.5 minutes
Krypton-76= 14.8 hours
Krypton-81= 2.1 x 10^5 years

Kr-73 = 23.625 sec

Kr-74 = 10.0625 minutes (603.75 seconds)

Kr-76 = 12.95 hours (777 minutes)

Kr-81 = 1.8375 * 10^6 years

Explanation:

What you do is multiply each of those amounts by 7/8.

27 * 7/8 = 189/8 = 23.625

23/2 * 7/8 = 161/16 = 10.0625

74/5 * 7/8 = 518/40 = 259/20 = 12.95

210000 * 7/8 = 1470000/8 = 183750 = 1.8375 * 10^6

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